Valorization of chitin derived N-acetyl-D-glucosamine into high valuable N-containing 3-acetamido-5-acetylfuran using pyridinium-based ionic liquids

Article abstract of DOI:10.1016/j.molliq.2021.115667

Chitin and its derivatives contain biologically fixed nitrogen elements, which can provide nitrogen sources for N-containing chemicals. Herein, a series of pyridinium-based ionic liquids were synthesized to directly catalyze the conversion of N-acetyl-D-glucosamine (NAG, the monomer of chitin) to 3-acetamido-5-acetylfuran (3A5AF). The yield of 3A5AF in 1-carboxymethyl pyridinium chloride ionic liquid reached 37.49%, without any additives. Using B₂O₃ and CaCl₂ as additives, the optimum yield increased to 67.37% at 180 °C in 20 min. In addition, HPLC-MS analysis has been utilized to elucidate the reaction mechanism. This research on turning “waste” into “wealth” opens up new ways for the utilization of biomass waste, which not only reduces environmental pollution but also has potential economic value.

Full text of DOI:10.1016/j.molliq.2021.115667

Journal of Molecular Liquids 330 (2021) 115667
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Short Communication
Valorization of chitin derived N-acetyl-D-glucosamine into high valuable
N-containing 3-acetamido-5-acetylfuran using pyridinium-based
ionic liquids
Hongjun Zang ⁎, Jing Lou, Shuolei Jiao, Huanxin Li, Yannan Du, Jiao Wang
State Key Laboratory of Hollow Fiber Membrane Materials and Processes, School of Chemistry and Chemical Engineering, Tiangong University, Binshuixi Road, Tianjin 300387, China.
a r t i c l e i n f o
a b s t r a c t
Article history:
Chitin and its derivatives contain biologically fixed nitrogen elements, which can provide nitrogen sources for
N-containing chemicals. Herein, a series of pyridinium-based ionic liquids were synthesized to directly catalyze
the conversion of N-acetyl-D-glucosamine (NAG, the monomer of chitin) to 3-acetamido-5-acetylfuran (3A5AF).
The yield of 3A5AF in 1-carboxymethyl pyridinium chloride ionic liquid reached 37.49%, without any additives.
Received 9 December 2020
Received in revised form 6 February 2021
Accepted 12 February 2021
Available online 16 February 2021
Using B
HPLC-MS analysis has been utilized to elucidate the reaction mechanism. This research on turning “waste” into
wealth” opens up new ways for the utilization of biomass waste, which not only reduces environmental pollu-
tion but also has potential economic value.
2 3 2
O and CaCl as additives, the optimum yield increased to 67.37% at 180 °C in 20 min. In addition,
Keywords:
Chitin
NAG
“
3-acetamido-5-acetylfuran
© 2021 Elsevier B.V. All rights reserved.
Pyridinium-based ionic liquid
1. Introduction
landfills and oceans, not only reduces environmental pollution but
also brings economic benefits.
Since the shortage of non-renewable fossil fuels, many efforts are
There have been some studies on the conversion of chitin and its de-
rivatives to platform compounds, such as 3-acetamido-5-acetylfuran
(3A5AF) [8–10], 5-hydroxymethylfurfural (HMF) [11–14], levulinic
acid (LA) [14–16], pyrrole [17] and so on. 3A5AF is a potential N-
containing platform compound, which can be used to prepare a series
of high value-added downstream products. For instance, the alkaloids
hyrtioseragamine A and B, pyrrolosine and the anticancer agent
proximicin A all contain 3A5AF fragments [6,18]. However, 3A5AF is
usually challenging to synthesize by conventional methods due to its
special molecular structure. Therefore, obtaining 3A5AF by direct con-
version from chitin and its derivatives is simpler and faster, which can
further simplify the synthesis steps of a series of bioactive molecules.
In 1984, Franich et al. [19] obtained 3A5AF for the first time by
degrading NAG under high temperature, though the yield was only
2%. The pyrolysis reaction at high temperature is complicated, with
many by-products, poor selectivity and high energy consumption,
which limits its research and application. Until 2012, Drover et al. [8]
found that [BMim]Cl ionic liquid can effectively catalyze the conversion
of NAG to 3A5AF, and the yield of 3A5AF can reach 25% under micro-
being made to find renewable alternative energy. Biomass, which is
called as “the fourth largest energy source” in the industry other than
coal, oil and natural gas, because it is abundant, renewable and widely
distributed in nature. Conversion of biomass into value-added platform
chemicals has become an important research area, as a way to alleviate
the energy crisis [1]. Currently, the utilization of biomass is mainly con-
centrated on woody biomass, such as lignin and cellulose [2]. However,
limited attention has been paid to ocean-based biomass [3].
Chitin, a classic example of ocean-based biomass, is the second-
largest abundant biomass in the world after cellulose, with a global an-
nual output of about 100 billion tons [4]. Research on the conversion of
chitin biomass was still in its infancy until the concept of “shell
biorefinery” was proposed [5]. One of the main reasons why chitin is
worthy of attention is that its composition contains 7 wt% of
biologically-fixed nitrogen [6]. It thus represents a potential raw mate-
rial for the production of N-containing chemicals which cannot be ob-
tained from lignocellulosic biomass. The industrial mass production of
chitin is carried out by extracting shellfish wastes from the fishing in-
dustry [7], such as shrimp and crab shells. Therefore, the conversion of
chitin biomass into valuable chemicals instead of pouring them into
3
wave heating. More surprisingly, the addition of B(OH) increased the
yield to more than 60%. At the same time, Omari et al. [9] reported
that with dimethylacetamide (DMA) as the solvent, the presence of
NaCl and B(OH)
yield of 58.0% under microwave irradiation. Chen et al. [3] found that
in the presence of [BMim]Cl, B(OH) and HCl, the yield of 3A5AF can
3
can catalyze the conversion of NAG to 3A5AF with a
⁎
Corresponding author.
E-mail address: chemhong@126.com (H. Zang).
3
https://doi.org/10.1016/j.molliq.2021.115667
167-7322/© 2021 Elsevier B.V. All rights reserved.
0

H. Zang, J. Lou, S. Jiao et al.
Journal of Molecular Liquids 330 (2021) 115667
reach 55.6% at a lower temperature (180 °C) under microwave condi-
tion. The above studies showed that boron and chlorine play a critical
role in the formation of 3A5AF. However, the reaction conditions of
the above experiments were relatively harsh. Although higher yields
were obtained under the microwave, those reaction conditions cannot
be widely used in industrial production. As a monomer of chitin, NAG
is relatively easy to convert into 3A5AF. However, there is still much
room for improvement in the yield of 3A5AF.
pyrrolidone (NMP) and Dimethyl sulfoxide (DMSO) were purchased
from Tianjin Damao Chemical Reagent Factory (Tianjin, China).
N, N-Dimethylacetamide (DMA) and N, N-Dimethylformamide (DMF)
were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. (Tian-
jin, China). Methanol (HPLC grade) was purchased from Tianjin
Kangkede Technology Co., Ltd. (Tianjin, China). Ethanol absolute was
purchased from Tianjin Huihang Chemical Technology Co., Ltd. (Tianjin,
China). Boric acid and all Lewis acids were purchased from Tianjin Sail-
boat Chemical Reagent Technology Co., Ltd. (Tianjin, China). Acetonitrile
(HPLC grade) was purchased from Tianjin Saifu Century Technology De-
velopment Co., Ltd. (Tianjin, China). Boron oxide and nicotinamide were
purchased from Tianjin Guangfu Fine Chemical Research Institute (Tian-
jin, China). Pyridine was purchased from Tianjin Rgent Chemical Co.,
Ltd. (Tianjin, China). Monochloroacetic acid, 2-Hydroxypyridine,
4-Aminopyridine, 4-Dimethylaminopyridine, 2-Methypyridinel were
purchased from Tianjin Heowns Biochemical Technology Co., Ltd. (Tian-
jin, China). All the chemicals were used as received.
In our laboratory's previous work, Wang et al. [20] used amino acid
ILs as catalysts to efficiently convert NAG into 3A5AF. The reaction was
heated at 180 °C in an oil bath to replace the microwave reactor, and the
yield of 3A5AF reached 52.61% in 10 min. Moreover, substituting micro-
wave conditions may increase the possibility of industrial application.
There have been some reports using ionic liquids (ILs) as solvents or
catalysts in biomass conversion [21–23]. The use of ILs conforms to the
trend of environmentally friendly catalytic systems. However, most of
them are currently only in the laboratory stage, and there is almost no
industrialization report. Besides, the types of ILs used in this field are rel-
atively single, and it is highly desired to explore new catalytic systems.
Pyridinium salts are important scaffolds for many natural and bio-
logically active products, which have a wide range of applications.
Pyridinium salts that are liquids at room temperature are called
pyridinium-based ILs (PyILs) [24]. Chinnappan et al. [25] observed
that compared with imidazolium-based ILs, PyILs had better catalytic
performance in organic conversion reactions. Subsequently, they used
pyridyl double cationic IL to catalyze the conversion of fructose to
HMF, and the yield reached 95% [26]. Taheri et al. [27] used PyIL/water
mixture to effectively dissolve cellulose, chitin and chitosan. Naz et al.
2.2. Synthesis of pyridinium-based ionic liquids
Synthesis of pyridinium-based ILs according to the method reported
in the literature [27] and developed in the experiment. For instance, the
synthesis steps of 1-carboxymethyl pyridinium chloride ([CMPy]Cl) IL
are as follows. Equimolar monochloro acetic acid and pyridine were
mixed in ethanol, and the mixture was stirred for 1.5 h in an ice bath.
After 1.5 h, the mixture was transferred to an oil bath at 80 °C, stirred
and refluxed for 60 min. Then, the yellowish precipitate was formed, in-
dicating that the structure of the ionic liquid has been synthesized. At
last, the precipitate was recrystallized in ethanol. The product was
dried under vacuum at 80 °C for 12 h to obtain the [CMPy]Cl.
[28] used PyIL-metal salt system to effectively deconstruct lignocellu-
lose and convert it into reducing sugars. Based on the above researches,
we applied PyILs to the conversion of chitin biomass.
1
In this paper, we used a series of PyILs as catalysts to convert NAG to
The [CMPy]Cl was characterized by H Nuclear Magnetic Resonance
( H NMR) spectroscopy (400 M, Bruker, Germany) and Fourier trans-
1
3
A5AF. Besides, we screened the effects of solvents and additives, and
optimized the amount of PyIL, reaction temperature and reaction time.
Under the optimal conditions, the yield of 3A5AF reached 67.37%. The
possible reaction process of converting NAG to 3A5AF was tested by
HPLC-MS.
form infrared (FTIR) spectroscopy (vertex80, Bruker, Germany). The
characterization results are consistent with those reported in the litera-
ture [29], proving that the [CMPy]Cl was successfully synthesized.
Other pyridinium-based ionic liquids were prepared in a similar
manner, using pyridine ring with different substituents to react with
monochloro acetic acid. As shown in Scheme 1, including 1-carboxymethyl-
2. Materials and methods
2-methyl pyridinium chloride ([CMMPy]Cl), 1-carboxymethyl-4-amino
2
.1. Materials
pyridinium chloride ([CMAPy]Cl), 1-carboxymethyl-4-dimethylamino
pyridinium chloride ([CMDAPy]Cl), 1-carboxymethyl-3-formamido
pyridinium chloride ([CMFPy]Cl), 1-carboxymethyl-2-oxhydryl pyridinium
chloride ([CMOPy]Cl), pyridinium chloride ([Py]Cl), and pyridinium
N-acetyl-D-glucosamine (NAG) and chitin were purchased from
Shanghai Aladdin Reagent Co., Ltd. (Shanghai, China). Methyl-2-
Scheme 1. Synthesis of PyILs.
2

H. Zang, J. Lou, S. Jiao et al.
Journal of Molecular Liquids 330 (2021) 115667
hydrogen sulfate ([Py]HSO
supplementary information.
4
). The characterization results are listed in the
mechanism by analyzing the intermediate products produced in each
reaction stage.
2.3. Conversion of NAG to 3A5AF
3. Results and discussion
In a typical reaction, NAG, PyIL, additives and solvent were mixed in
3.1. Catalyst and solvent screening
a round bottom flask, and then the mixture was stirred in an oil bath at a
certain temperature for a certain time. When the reaction was over, the
mixture was immediately moved to an ice water bath to cool down,
then diluted with 50% methanol aqueous solution. In order to remove
solid residues, the solution was filtered through a 0.22 μm PTFE filter.
The sample solution was determined by High Performance Liquid Chro-
matography (HPLC). Three parallel experiments were conducted to cal-
culate the average yield of 3A5AF.
We screened the catalytic effects of eight different cations and an-
ions PyILs. At the beginning of this study, NMP was used as the solvent.
100 mg NAG, 100 mg PyIL and 10 mL NMP were mixed, and the mixture
was stirred in an oil bath at 180 °C under atmospheric reflux for one
hour. Under the same experimental conditions as above, the results of
3A5AF yield that obtained from this reaction using different PyILs are
listed in Table 1.
The best performance was obtained in the presence of [CMPy]Cl, and
the yield of 3A5AF reached 37.49% (entry 1). The anions in ionic liquids
2.4. Determination of the products
are critical to the reaction, especially the chloride ions [6]. Compared
−
HPLC was used for quantitative analysis of 3A5AF. HPLC instrument
with other ILs containing chloride ions, the IL containing HSO
4
anion
(
LC3000, Beijing Chuangxin Tongheng Science and Technology Co., Ltd.)
obtained lower yield, only 6.36% (entry 8). In addition, by comparing
[CMPy]Cl and [Py]Cl, we can find that the carboxyl linked to the nitrogen
atom in pyridine greatly improved the catalytic effect (entries 1 and 7).
The use of pyridine as a cation has achieved a significant catalytic effect.
Unfortunately, the different substituents on the pyridine ring did not
significantly promote the reaction (entries 2–6). Besides, we measured
the pH value of the PyILs. All of them have strong acidity and [CMPy]Cl
has the strongest acidity, indicating that the acidic environment is con-
ducive to NAG conversion. However, when using [CMPy]Cl to degrade
chitin, the yield was only 1.21% (entry 9). Due to its high degree of po-
lymerization, chitin is more difficult to degrade than NAG.
with a Kromasil C18 reversed phase column (5 μm, 250 mm × 4.6 mm)
and the temperature of the column was maintained at 30 °C. The UV de-
tector wavelength was set to 230 nm. The mobile phase consisted of
methanol and water (40/60 v/v) with a flow rate of 0.5 mL/min, and
the sample injection volume was 20 μL. The peak of 3A5AF as the
main product in the HPLC spectrum appeared at 17 min. The concentra-
tion of 3A5AF was quantified by a standard curve obtained with a stan-
dard substance of known concentration. The standard curve was Y =
4
detected by HPLC analysis, and X was the concentration of 3A5AF in
the sample.
2
05,943× + 260,422, (R = 0.9998), where Y was the peak area data
The four solvents selected include DMA, DMF, DMSO and NMP,
which have been reported to be frequently used for sugar dehydration
[30–34] and chitin dehydration [10]. NAG and PyIL were dispersed in
High Performance Liquid Chromatography-Mass Spectrometry
HPLC-MS, Xveo G2Q-Tof, Waters, America) can explore the reaction
(
Table 1
Effect of different PyILs on the conversion of NAG into 3A5AF under normal atmospheric reflux conditions.
Entry
1
Catalyst
Substrate
NAG
pH value of PyILs
1.17
3A5AF yield (mol %)
37.49
2
3
NAG
NAG
1.38
2.86
27.66
34.07
4
NAG
1.82
34.45
5
6
NAG
NAG
1.62
2.01
13.23
14.18
7
NAG
2.50
17.45
8
9
NAG
1.31
1.17
6.36
1.21
chitin
Experimental conditions: Substrate, 100 mg; PyIL, 100 mg; NMP, 10 mL; temperature, 180 °C; reaction time, 60 min.
3

H. Zang, J. Lou, S. Jiao et al.
Journal of Molecular Liquids 330 (2021) 115667
Fig. 1. Effect of PyILs in different solvents on 3A5AF production under normal atmospheric reflux conditions. Reaction conditions: NAG, 100 mg; PyIL, 100 mg; solvent, 10 mL; reaction
temperature, 180 °C; reaction time, 60 min.
these four solvents at 180 °C for one hour, and the results are shown in
Fig. 1. From above DMA and NMP are all suitable solvents for NAG con-
version, but NMP appears to be most effective. Chen et al. [10] proposed
that DMA and DMF were the best solvents for NAG dehydration, while
NMP was the best solvent for chitin dehydration. It is speculated that
the reason may be related to the structural difference between chitin
Scheme 2. Possible reaction pathways.
4

H. Zang, J. Lou, S. Jiao et al.
Journal of Molecular Liquids 330 (2021) 115667
Fig. 2. Single additive screening for NAG conversion into 3A5AF under normal atmospheric reflux conditions. Reaction conditions: NAG, 100 mg; [CMPy]Cl, 100 mg; additive, 100 mg; NMP,
0 mL; reaction temperature, 180 °C; reaction time, 60 min.
1
Fig. 3. Combinational additives screening for NAG conversion into 3A5AF under normal atmospheric reflux conditions. Reaction conditions: NAG, 100 mg; [CMPy]Cl, 100 mg; B
2 3
O ,
100 mg; Lewis acid, 100 mg; NMP, 10 mL; reaction temperature, 180 °C; reaction time, 60 min.
Fig. 4. Effect of reaction temperature on 3A5AF production under normal atmospheric reflux conditions. Reaction conditions: NAG, 100 mg; [CMPy]Cl, 100 mg; B
2 3 2
O , 100 mg; CaCl ,
100 mg; NMP, 10 mL; reaction time, 60 min; reaction temperature 160–210 °C.
and NAG. The molecular size of DMA and DMF is relatively small, which
is more conducive to the solute-solvent interaction in NAG conversion.
Obviously, this conclusion is inconsistent with the results of our solvent
screening. However, the solvent effect is a complex problem affected by
many factors, among which the interaction between solvent and cata-
lyst is the most important factor, and the catalytic performance is
5

H. Zang, J. Lou, S. Jiao et al.
Journal of Molecular Liquids 330 (2021) 115667
Fig. 5. Effect of the IL/NAG mass ratio on 3A5AF production under normal atmospheric reflux conditions. Reaction conditions: NAG, 100 mg; [CMPy]Cl, 50 mg ~ 500 mg; B
2 3
O , 100 mg;
CaCl , 100 mg; NMP, 10 mL; reaction temperature, 180 °C; reaction time, 60 min.
2
Fig. 6. Effect of reaction time on 3A5AF production under normal atmospheric reflux conditions. Reaction conditions: NAG, 100 mg; [CMPy]Cl, 100 mg; B
2 3 2
O , 100 mg; CaCl , 100 mg; NMP,
10 mL; reaction temperature, 180 °C; reaction time, 10 min ~ 100 min.
strongly influenced by solvent effects [35]. Therefore, we consider that it
is due to the stronger interaction between NMP and PyILs, not just
judged by the interaction between the substrate and the solvent. On
the other hand, Pumrod et al. [36] found that NMP can promote the
isomerization of fructose to fructofuranose, thereby promoting the for-
mation of HMF. The conversion process in this work also has an isomer-
ization process (see Scheme 2), so NMP may have a good effect by
promoting molecular isomerization. In addition, NMP can also be used
as an aqueous phase to prevent humin formation and reduce the forma-
tion of other by-products [36]. From the standpoint of solvent effects,
we hypothesize that the synergistic effect between solvent molecular
and PyILs could result in a higher yield of 3A5AF.
catalytic performance [3,10]. Therefore, we used boron-based com-
pounds and Lewis acids as additives, and the results are shown in Fig. 2.
Among them, the catalytic effects of B
lent, and both increase the yield of 3A5AF to a certain extent. We infer
that B can react with trace water in the reaction system (from the-
solvent or water produced by the dehydration of NAG) to form
B (OH) . Hence, the catalytic effects of B and B(OH) were almost
equivalent.
Among a series of Lewis acids, CaCl
Similarly, the presence of MgCl , AlCl and CrCl
, NiCl , KCl and FeCl
2 3 3
O and B(OH) were equiva-
2 3
O
3
2
O
3
3
2
gave the highest 3A5AF yield.
also slightly increased
had a negative ef-
2
3
3
2
the yield. However, BaCl , ZnCl
fect on the reaction.
2
2
2
The reason, first of all, from the perspective of Lewis acid's promot-
ing effect, it is mainly divided into two aspects. Above all, Lewis acid
can promote some isomerization reactions [37,38], so it is likely to pro-
mote the enol isomerization process of this reaction. Secondly, Lewis
acid can play a strong dehydration effect. However, while accelerating
the dehydration reaction, it also accelerates the carbonization of the
substance, resulting in the formation of undesired humin [37]. The de-
position of humin will inevitably affect the catalyst's activity and the
progress of the reaction, thereby negatively affecting the reaction.
Therefore, we hold that the relative strength of promotion and negative
effects can explain the catalytic performance of Lewis acids in this study,
as each catalyst has its own optimum operating conditions.
3
.2. Additives screening
[CMPy]Cl was selected as the catalyst in the following experiments
because it had the highest yield in the above experiment. Herein, addi-
tives were screened to increase the target product yield. The following
experiments explored single additives and additive combinations.
3.2.1. Single additive screening
Boron-based compounds have the advantages of non-corrosion,
non-toxic, non-metal, good biocompatibility, cheap and easy to obtain,
so they have a large space for utilization in biomass conversion. Chen
et al. [10] found that B(OH)
of chitin to produce 3A5AF, chitin and B(OH)
3
can effectively promote the degradation
can form a boron complex
3
3.2.2. Combination of additives
to promote the formation of 3A5AF. Lewis acids are often used to cata-
lyze the preparation of 3A5AF from chitin biomass and show certain
For the purpose of obtaining higher yields, combinations of additives
were tried. B O is the best single component additive, and it is more
2 3
6

H. Zang, J. Lou, S. Jiao et al.
Journal of Molecular Liquids 330 (2021) 115667
Fig. 7. HPLC-MS spectra of the reaction system. A, reaction for 0 min; B, reaction for 7 min; C, reaction for 10 min; D, reaction for 20 min.
7

H. Zang, J. Lou, S. Jiao et al.
Journal of Molecular Liquids 330 (2021) 115667
Fig. 7 (continued).
8

H. Zang, J. Lou, S. Jiao et al.
Journal of Molecular Liquids 330 (2021) 115667
environmentally friendly and cheaper than B(OH)
binations consist of B and another additive.
As shown in Fig. 3, the combinations of B
CaCl can increase the yield. The combination of B
3
. Therefore, the com-
characteristic peak 2 (t
proves the existence of “IV”.
“IV” formed an aldehyde structure (V) through enol isomerization,
and then “V” removed one molecule of water to form “VI”. “VI” corre-
R
= 2.15 min, Mr. = 263) in Fig. 7 (D), which
O
2 3
2
O
3
and AlCl
3
, CrCl
3
and
2
2
O
3
and CaCl
2
per-
formed best, the yield of 3A5AF reached 66.97%. For other Lewis acids,
the 3A5AF yield increased to a certain extent due to the addition of
R
sponds to the characteristic peak 3 (t = 1.60 min, Mr. = 184) in
Fig. 7 (B, C, D), which proves the existence of “VI”.
B
2
O
3
, but it was lower than that of using B
that the combined additives produced synergistic effects. We found
that CaCl has good catalytic properties in many other studies. For in-
stance, Nan et al. [39] found that CaCl as a catalyst can play various
roles such as dehydration, deamination, decarboxylation and dehydro-
genation. In the work of Lin et al. [40], CaCl can promote the ionization
of its substrate and chelate with the substrate. In our work, CaCl may
2
O
3
alone. It is speculated
Subsequently, “VI” removed one molecule of water to form
3A5AF.The characteristic peak 4 (t = 2.30 min, Mr. = 166) in Fig. 7
R
(B, C, D) corresponds to 3A5AF, which proves that the product 3A5AF
was obtained.
2
2
2
4. Conclusions
2
play a role in strengthening dehydration and promoting enol
isomerization.
We have synthesized a series of pyridinium-based ionic liquids and
applied them to the conversion of NAG to 3A5AF. Under the synergistic
catalysis of [CMPy]Cl and additives, the yield of 3A5AF under optimal
conditions can reach 67.37%. We analyzed the reaction mechanism of
NAG to 3A5AF through HPLC-MS test. This research made a break-
through in the yield of 3A5AF and enriched the catalytic system of chitin
biomass. However, the research on the conversion of chitin biomass into
N-containing compounds is still in its infancy, and there are many
aspects that require our efforts. This work still has problems that need
improvement. For instance, our catalytic system is only suitable for
the degradation of chitin monomer, and more in-depth researches are
needed on the degradation of chitin. In future work, we should explore
more low-cost, green and recyclable catalysts, such as solid acid cata-
lysts, which are easy to separate and recycle to reduce costs. In addition,
there are few reports on the preparation of other types of N-containing
compounds by chitin biomass, so more other types of products should
be explored to broaden the application range of chitin in the field of
N-containing chemicals manufacturing.
2 3 2
In summary, we use B O and CaCl as a combination additive to co-
catalyze the conversion of NAG to 3A5AF with [CMPy]Cl.
3.3. Optimization of reaction conditions
The influences of temperature, time and IL concentration on the re-
action were examined.
The temperature factor was investigated first, and the reaction was
carried out in the temperature range of 160 °C to 210 °C. As shown in
Fig. 4, the yield increased with increasing temperature in the range of
1
60 °C to 180 °C, and reached the highest value at 180 °C. However,
the yield began to drop at 190 °C, which may result from the thermal de-
composition of 3A5AF. And there may be another reason that IL became
unstable and decomposed at high temperature, affecting the catalytic
effect. Therefore, the optimal temperature for this reaction was 180 °C.
And then, the concentration of the IL was optimized by conducting
experiments at the mass ratio of [CMPy]Cl to NAG from 0.5 to 5
(Fig. 5). It can be seen that with the increase of [CMPy]Cl concentration,
the yield of 3A5AF did not change much. When the mass ratio was 2, the
highest yield of 3A5AF reached 68.82%, slightly higher than the yield
when the mass ratio was 1. From the perspective of reducing the
amount of IL, the mass ratio of 1 was selected as the best feeding ratio
in the subsequent experiments.
Author statement
Hongjun Zang: Conceptualization, Methodology, Writing - Review
& Editing, Project administration, Supervision, Revising manuscript.
Jing Lou, Methodology, Software, Validation; Formal analysis, Inves-
tigation, Resources, Data Curation, Writing - Original Draft，Revising
manuscript.
Finally, to determine the best reaction time, the experiments carried
out for 10 min, 15 min, 20 min, 30 min, 40 min, 60 min, 80 min and
1
00 min. As shown in Fig. 6, the results showed an upward trend in
the range of 10–20 min and then stabilized. With the extension of reac-
tion time, the yield dropped slightly, which may be due to the decompo-
sition of 3A5AF. Therefore, the best reaction time was determined to be
Shuolei Jiao, Methodology, Software, Validation;
Huanxin Li, Resources, Software,
Yannan Du, Investigation.
20 min.
Jiao Wang, Formal analysis.
3.4. Possible reaction pathways
The reaction mechanism was determined by HPLC-MS, and the reac-
Declaration of Competing Interest
tion pathway was inferred from the test results, as shown in Scheme 2.
Fig. 7 shows the HPLC-MS analysis results when the reaction time
was 0 min, 7 min, 10 min and 20 min respectively. [CMPy]Cl dissolved
in NMP and ionized Cl . Cl broke the intramolecular and intermolecu-
lar hydrogen bonds in the NAG molecular structure, and then formed a
The authors declared that we have no conflicts of interest to this
work. We declare that we do not have any commercial or associative in-
terest that represents a conflict of interest in connection with the work
submitted.
−
−
new hydrogen bond with -OH. Therefore, the characteristic peak 1 (t
retention time) = 0.82 min, Mr. (Relative molecular mass) = 256) in
Fig. 7 (A) corresponds to “I”, which is the NAG-Cl complex.
R
(
Acknowledgements
−
NAG opened its ring under acidic conditions and changed from
hemiacetal form to open chain aldehyde form “II”. Subsequently, the
open chain aldehyde structure was isomerized to form an enol interme-
diate. And then, the lone pair of electrons on the O atom connected to
The following funding sources are correct. National Natural Science
Foundation of China (No. 21406166) and Natural Science Foundation
of Tianjin Science and Technology Correspondent Project (No.
18JCYBJC87200).
the C
the C
5
position of the enol intermediate attacked the double bond at
position to form a new C \\O bond, forming a five-membered
2
2
Appendix A. Supplementary data
ring intermediate (III).
The boron‑oxygen bond in B
of “III” to form a boron complex (IV). “IV” corresponds to the
2
O
3
cooperated with the hydroxyl group
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2021.115667.
9

H. Zang, J. Lou, S. Jiao et al.
Journal of Molecular Liquids 330 (2021) 115667
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